Robertson (1959) was first proposed the concept of unit membrane model. According to Robertson, the unit membrane consists of a bimolecular lipid leaflet sandwiched between outer and inner layers of protein, organized in the pleated sheet configuration, Protein-phospholipid -phospholipid - protien.
Each unit membrane is 75A˚(65−85A˚) thick. Such an arrangement was presumed to be basically the same in all cell membranes.